As is widely known, TIG (Tungsten Inert Gas) welding is a welding method wherein a base metal and a filler wire are melted by means of an arc discharge generated between the base metal and a tungsten electrode of a welding torch, and an insert gas such as argon gas is blown onto the weld pool to provide an atmosphere of inert gas around the weld pool. The welding torch employed in TIG welding is either a single electrode type in which one electrode is used, or a twin electrode type in which two electrodes are used.
In the case of TIG welding employing the twin electrode type welding torch, a phenomenon occurs where the arcs generated at each electrode are electromagnetically attracted and concentrated in the welding groove, thus narrowing the range over which the welding groove is melted. In order to rectify this situation, the welding torch is displaced orthogonal to the line of the weld. On the other hand, for example in Japanese Unexamined Patent Application, First Publication No. 2000-079477, there is disclosed an example of a welding method wherein a twin electrode type welding torch is employed. This method applies to one electrode a welding current which varies in pulses at a predetermined period, and furthermore, applies to the other electrode a welding current which varies in pulses at an opposite phase to the variations of the welding current of the one electrode. In this manner, by alternately varying the welding current supplied to each electrode, the electromagnetic attraction between the arcs generated at each electrode is weakened, and concentration of the arc from each electrode in the welding groove is alleviated. As a result, the range over which the welding groove is melted is increased, and the penetration of the welding groove can be stabilized.
However, in the case of welding at an attitude other than downwards, the force (the retaining force) supporting the weld pool against the force of gravity is comprised primarily of the surface tension of the weld pool, and the arc pressure, and the weld pool is held at the welding groove by the balance of these forces. If the welding attitude is changed to other than downwards, for example from downwards to upwards, a phenomenon occurs wherein the balance between the force of gravity and the retaining force is lost and the molten metal dribbles from the weld pool. Therefore, welding efficiency is kept low, and the weld pool must be reduced in size. On the other hand, methods where the arc pressure is increased by increasing the welding current in order to inhibit dribbling of the molten metal have been considered. However if the arc pressure is concentrated at a specific site in the weld pool, a localized depression is formed in the surface of the weld pool. Therefore it is impossible to effectively inhibit the aforementioned dribbling phenomenon. Hence, in order to effectively inhibit the dribbling phenomenon by increasing the arc pressure it is necessary to disperse the arc pressure over a wider range of the weld pool.
Moreover when inhibiting the dribbling phenomenon by increasing arc pressure, it is undesirable to bring about an increase in arc current. Arc pressure is primarily attributable to the flow of arc plasma towards the weld pool, and increases with welding current. However, increasing welding current also increases heat input into the weld, promoting melting of the base metal and resulting in an increase in the size of the weld pool. Therefore, the supply of filler wire must be reduced by that amount, so that welding efficiency is reduced. Consequently, in order to increase the efficiency of welding work, it is desirable to be able to increase arc pressure without increasing welding current.